4-6-6-4 — The Challenger Survivors

The Union Pacific Railroad has been on long journey to find the ideal
road locomotive that could handle the grades found on its transcontinental
line. The UP was making due with multiple lash-ups and helper engines,
but the extra locomotives, crews, and fuel was expensive.

To address this problem, UP worked with the American Locomotive Company
to design a new engine. They took the largest steam locomotive that
ALCO had, the 4-12-2, and started reworking it. The biggest change was
to split the 12 wheels into two groups of 6 wheels. The front 6 drivers
and 4 pilot wheels would be on a sub-frame that was able to swivel
independent of the rest of the locomotive. This is a feature called
articulation. Other changes included going from 3 cylinders to 4
cylinders, a larger firebox, larger boiler, and slightly higher steam
pressure.

The resulting machine was a winner. UP tested the new 4-6-6-4 engine
on a section of track in Utah over the Wahsatch Range. The engineer
commented that would be a challenge for any locomotive, and the name
stuck. UP officially called the machine the Challenger.

A total of 252 Challengers were built between 1935 and 1943. UP received
the most, 105. Other railroads that used the Challenger include the
Clinchfield, Delaware & Hudson, Denver & Rio Grande Western,
Great Northern, Northern Pacific, Spokane Portland & Seattle, the
Western Maryland, and the Western Pacific.

As WWII progressed, the Challenger and its brother the Bigboy, were
workhorses moving war materials across the US. But as soon as the war
ended, diesel locomotives started becoming available. The diesel
locomotives were so much more efficient that the steam machines were
quickly doomed. The Challengers started being retired in the late
1950s, some after only 10 years of service. All were gone from UP
service by 1962. Of those 252, only 2 survive today. UP #3977 is on
static display, and UP #3985 remains in active service with the UP
Heritage Fleet.